A batter hits a foul ball that goes straight back, hitting the catcher. Their heads are well-protected, but try getting a baseball change direction on you at a 90 miles an hour and hit you in the head. You'll probably have your mask come off and need a couple of seconds to regroup. Even though they don't have to, the home plate umpire will walk to the pitcher to give him a new ball in order to give the catcher time to regroup.
This works the other way around: if an umpire gets a foul ball hit his mask, the catcher will walk up to the pitcher and have a conversation. Maybe they talk about what strategy they will use against the current batter, maybe they won't, but the purpose of the visit is to give the umpire a break. Umpires and catchers are in it together, so they take care of each other. Every now and then you'll see one tap the other for thanks.